Overlay measurement technologies, especially scatterometry, require that the cells which make up the target (e.g., SCOL (scatterometry overlay) targets), are all perfectly periodic with the same pitch. Any breakdown of this requirement involves a degradation of accuracy and TMU (total measurement uncertainty). Specifically, if the breakdown of periodicity is such that a position on the SCOL cell exists, which generates a pupil signal consisting of a symmetry breaking that is only due to the SCOL cell's offset, then only at that special position the SCOL inaccuracy is zero. Obviously it is not guaranteed that this ‘special’ position exists in any useful sense (i.e., that the position is known and that it is surrounded by regions which are large enough to enable locating the position in spite positioning errors).
Furthermore, any misalignment of the illumination system from that special position causes further inaccuracy, due to the following physical process explanation. First, considering as an illustrative example a perfectly periodic target of a typical pitch P=600 nm which is illuminated by a monochromatic light of wavelength 400 nm, the only diffraction orders which enter the collection pupil plane are the −1st, the 0th, and the +1st diffraction orders. Also, these orders are completely separated in pupil space and their edges are infinitely sharp. If the spot now moves across the target, each order carries its own pupil phase (which is proportional to the spot movement parameter, x, to the diffraction order index, n, and inversely proportional to the target pitch). As the pupil detector (e.g., a CCD) measures intensity, this phase information is washed out and no knowledge about the spot movement is carried in the pupil SCOL signals, which means that the measured overlay is perfectly independent from the spot position (as it should be on a perfectly periodic SCOL target). Similarly in the case of metrology tool defocus, the defocus-related phases are washed out.
Typically, approaches to measuring targets which are not perfectly periodic can be divided into two approaches. A first approach is disclosed by WIPO Patent Publication No. 2014004564 which includes averaging over a variety of spot positions to average out the interference terms. A second approach is to select target and measurement designs in which the breakdown of periodicity, as sensed by the metrology tool, is small. Examples for such designs include increasing target size, modifying the process flow, picking illumination conditions like wavelength and polarization that are insensitive to the target noise. However, these methods have various shortcomings. The first method assumes that the length scale of the target noise is either very much smaller than the averaging length scale or very much larger; an assumption which may not hold in reality. Importantly, the quantitative knowledge of the length scale may be absent. Also, the averaging length scale is bounded from above by the SCOL cell's size thereby forming an upper bound to the effectiveness of the averaging. The success of the second method is predicated on the assumption that the sought combination of an optimal target design and illumination conditions indeed exists. Besides, it burdens the metrology tool design, which, to allow flexibility in illumination conditions, must be very diverse (e.g., contain many possible wavelengths).